MAP PROJECTIONS

A map is not a simple drawing
of the earth’s surface, but its model obtained on the basis of certain
mathematical laws. First, we pass from the physical earth’s surface onto the
mathematical surface – rotational ellipsoid or sphere. This transfer is realised
by means of orthogonal projection of physical surface points onto the
mathematical surface by using geodetic control network enabling correct
geographic location and orientation of map contents within the frame of some
co-ordinate network. Afterwards follows the transfer from the rotational
ellipsoid surface or sphere into the plane. These projections are called map
projections and are the subject of the theory of map projections.

Map projections have been developing
parallel with the development of map production and cartography in general. The
development of many sciences, technical achievements and the needs of everyday
life have gradually initiated wider and wider demands for the production of
various topographic and thematic maps in various scales and for various
purposes, which requested continuous growth of map projections and improvement
of mathematical basis of maps.

The various systems of map projection employed can be expressed as
mathematical formulae. Every point on the earth’s surface has a unique location
given by its geographical co-ordinates (latitude and longitude). A systematic
map projection takes those geographical co-ordinates and translates them into
Cartesian co-ordinates on the map by means of a mathematical formula. Before
the advent of rapid computing power, projecting a map required a great deal of
time to calculate, but computer programs are now readily available which allow
even very complex map projections to be carried out rapidly.

In converting the 3-D earth to Cartesian co-ordinates, the fixed relationship
between all points on the globe cannot be kept and distortions have to be
introduced. A map projection may distort angles, areas or distances; any map
projection cannot keep more than one of angle, area or distance correct.

Map projections fall into one of a number of groups, but there are four main
groups of projections: equal area or equivalent (which keep areas correct),
equal distance (or equidistant, which preserve distances in a particular direction, e. g. along the meridians), equal angle
(also called conformal which keep angles correct), and hybrid or miscellaneous
(where angles, distances nor areas are correct, but area and angle distortions can be somehow acceptable).